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Levering GPGPU and OpenCL technologies for natural user interfaces

YOUi Labs, experts in developing Natural User Interfaces (NUI) for embedded platforms, examines in a new white paper how both General Purpose Graphics Processing Units (GPGPU) and The Khronos Group’s OpenCL specification can be used in a NUI engine to best utilize the available hardware for efficiency and improved user experience.

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AbstractNatural User Interfaces (NUIs) are vastly more complex than traditional graphical user interfacesand require large computational power to provide an immersive experience for the user. We willexamine NUI improvements and design challenges over traditional Graphical User Interfacesand the associated computational complexities encountered during implementation. In order tomaximize the hardware’s capability for a NUI, making use of available Graphics Processing Unit(GPU) cycles to complement the Central Processing Unit (CPU) is crucial. This process of usingGPUs as General Purpose Graphics Processing Units (GPGPU) has been traditionally limited todesktop computing platforms, but as portable devices are becoming more powerful,encompassing multiple-core CPU and GPU elements, implementation becomes crucial forefficient use of the hardware’s capability. When effectively utilizing both the GPU and CPUcycles a smooth, fluid experience can be maintained, as well as optimization for best case powerconsumption. This can be very challenging due to platform constraints, differing CPU / GPUarchitectures, implementation complexity and cost of integration. We will examine a few aspectsof a NUI that could benefit from GPGPU computing and the associated implementation benefits. 3

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GPGPU“GPGPU stands for General-Purpose computation on Graphics ProcessingUnits. Graphics Processing Units (GPUs) are high-performance many-coreprocessors that can be used to accelerate a wide range of applications.” -http://gpgpu.org/ NUI“A natural user interface is a user interface designed to use natural humanbehaviors for interacting directly with content” -http://nui.joshland.org“A NUI relies on a user being able to carry out relatively natural motions,movements or gestures that they quickly discover control the computerapplication or manipulate the digital content.” -http://nuigroup.com“We started with Command-line Interfaces in the 60s, and then moved to GUIsin the 80s, and we have just entered into the next 25 year evolution - the era ofthe NUI” - Jason Flick, CEO, You i Labs. 4

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Utilizing GPGPU for Natural User Interfaces This paper will cover areas where General Purpose Graphics Processing Unit (GPGPU)functionality could be implemented in a NUI engine to best utilize the available hardware forefficiency and improved user experience. Both emerging trends and common optimization areaswill be discussed. Optimizations particular to the You I Labs uSwish framework will also bereviewed. Although the main focus will be on mobile devices, the techniques discussed caneasily be applied to other computing platforms. It is assumed that the reader is knowledgeable onthe basic concept of heterogeneous and parallel computing. The actual software implementationsusing OpenCL or OpenGL shaders is outside the scope of this paper and will not be discussed. NUI Background In traditional graphical user interfaces (GUI) most of the interactive elements andcontrollable parts in the interface are static until acted upon. We are all used to this interactionwhen using a traditional desktop computer environment. The mouse-and-keyboard driveninterface forces the user to navigate within the computer’s environment for interaction andselection. Most of the information or content is provided to the user in separate applications thatare simply launched from the GUI. As modern devices migrate to using touch screens, gesture,and other input methods the user interface must now instead navigate to the user’s requests –static icons are now scrollable to navigate virtual screen space and must react in a natural way toprovide appropriate feedback. In Natural User Interfaces (NUI) the user should experienceinteractions with the device that are natural and responsive. Actions such as sliding a finger 5

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across the display must be accurately translated into user requests for motion (list navigation),selection (initiate an event or action), or other potential interpretations. Once the action has beendecided, the natural flow of response must happen: 1: Real world physics style response 2: Immediate feedback to the user as acknowledgement of the request 3. Transitions that help to keep things in context 4: Preparation for next user interaction with minimal or no delayPerforming these tasks on modern interfaces can be quite costly in terms of CPU bandwidth.Consider a list style control of recently received emails. In a NUI environment the user wouldinteract directly with the list itself and their motions would be interpreted, such as slidinggestures to navigate the list. The interaction experience is natural and pleasing to the user. This isa crucial difference from traditional GUIs where a separate scrollbar, usually to the far side ofthe list, would need to be manipulated by the user in an unnatural opposite direction of the listmotion. To ensure a natural user response, the NUI would calculate the physical accelerationimplied by the user’s motion and provide a real-time update of the displayed email content as thelist scrolls to a natural feeling friction stop. The update process must achieve, at the very least, 24updates per second to appear natural and fluid. Modern NUIs normally target 60 frames persecond (FPS) for optimal user response. While performing these updates of both content andvisual components, the NUI must also continuously update the physical properties in response toother potential inputs from the user without any discernible delay. This results in a naturalfeedback response to the user. 6

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Due to the complexity in providing this fluid flow of content many NUI devices incorporateGraphics Processing Units (GPU) to provide high performance rendering freeing the CPU tohandle the user input, content control, and other aspects of the NUI environment. However, inmost cases the GPU is utilized only for visual performance enhancement and relatively under-utilized in the normal NUI environment. Many of the available mobile GPUs are capable of veryhigh performance polygon rendering that applies directly to 3D video games whereby hundredsof thousands of polygons would be rendered at high frame rates. When applied to current NUIenvironments which incorporate perhaps only hundreds of polygons, the GPUs are typically idlewhile the CPU is in constant use performing background system tasks. Due to the parallel natureof GPU design, making use of their computing power for other tasks in a NUI environment is anoptimum application for GPGPU enhancement.It is very important to understand that the demands of a NUI expand beyond just user inputs andfeedback to the handling of information and content, which should flow naturally to the user.More information is being provided within the context of the user interface directly rather thanthrough auxiliary programs, requiring more complexity and interactivity between the interfaceand user. In traditional GUI design, content for applications exists in silos that need only beaccessed and updated when in direct user focus. A common example is an email applicationwhere the content is only retrieved and presented in the context of the application itself. Beforerunning the application, the content contained may be out of date, incorrect or simplyinaccessible to the user. Having seamless content flow is crucial in creating a natural interactionexperience. This adds tremendous complexity as applications must freely pass, update andmaintain content information in the background of the NUI at all times. 7

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GPGPU considerations in NUI applications Unlike traditional CPU architecture, GPU design incorporates a Single Instruction MultipleData (SIMD) style architecture which works exceptionally well with large data sets usingidentical computations. Applying this technology for tasks in a NUI environment on mobiledevices has the following challenges: 1. The data sets for common NUI operations are relatively small 2. Visual effects are still the largest demand on the GPU 3. Complexity of some calculations is not feasible or efficient on GPU architecture 4. Lack of access to functional hardware components either through open standards such as OpenCL or proprietary APIs 5. Physical limitations in the device platform, such as OpenGL 2.0 compliant shader support and Frame Buffer Object (FBO) support with extensions applicable to GPGPU style implementation for creating computational blocksIt is also important to note that many other uses for GPGPU functionality are outside the scopeof the actual NUI itself: 1. Computer vision or OpenCV based intelligent image processing 2. Gesture tracking and interpretation 3. Live content decoding or parsing, such as video or audio content streams, especially for new formats not currently supported directly by the device hardware 8

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4. External device control or data processing, such as video camera image data compression or conversionWe need to consider multiple factors for utilizing GPGPU computing effectively within a NUIenvironment. We must also share the resources with the normal graphics computations as well aspotential GPGPU gains realized by external elements such as computer vision or gesturetracking. In addition consider the CPU and GPU bandwidth and requirements by otherapplications or external elements (such as networking). Simply stated, we cannot arbitrarilythrow computation segments at the GPU and expect improved performance. Efficient use of theGPU for computational support in parallel with CPU based computation to obtain the bestbalance between overall performance and availability for other elements is the key to success.Mobile applications should also consider power consumption and the associated costs ofbringing the GPU out of standby (if supported) versus dynamic CPU manipulation for powercontrol. Many modern mobile CPUs support dynamic frequency control for reducing powerconsumption when idle. We need to consider the consumption differences between pulling theCPU to full rate to process data versus the equivalent GPU power costs for the same operations.The GPU will prove less efficient than the CPU in certain circumstances such as very limiteddata sets where the overhead costs of preparing the hardware exceed the computational time onthe CPU. Managing the best power balance for computational selection is a complex part ofutilizing GPGPU for NUI enhancement (this limitation and potential solutions will be addressedin detail in a future white paper). This lends itself to increased code complexity as both CPU andGPU based versions of each supported algorithm must exist and the choice must be madedynamically based on content as to which resource is utilized. This issue will be minimized in 9

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the future through proper OpenCL implementations where the computational device being usedis abstracted from the algorithm code, allowing for shared code between the architectures. Wemust also consider memory sharing and potential for blocking operations between processors.Utilizing multi-core CPU as well as GPU devices adds to this complexity since the architectureof the devices must be understood to make optimum use of the computing power whileaddressing limitations such as shared resources. Proposed uses for GPGPU in NUI applications Several areas of NUI design are directly suited to implementation in GPGPU form. Our“Top 10”, in no particular order, are: 1. Visual features not traditionally handled by the GPU, such as font rendering 2. Layout of NUI elements and content frames 3. Enhanced Visual features 4. Sorting of data or other data handling services 5. Compression / decompression of content stores 6. Physical properties of objects – reactive physics effects processing 7. Complex animation or motion schemes for NUI visuals to appear more natural 8. Handling streaming NUI content 9. Real time security encryption / decryption 10. You I Labs uSwish NUI engine specific featuresLet’s consider each of these potential applications: 10

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Visual features not traditionally handled by the GPUMaking use of available GPU cycles for CPU tedious tasks such as font rendering or layoutcalculations provides a good starting point for NUI GPGPU optimization. Early mobile devicesused bitmap based fonts to remove the complexity and performance cost of dealing with vectorfont data. This solution had quality and expandability limitations that were reasonably acceptableat the time, but with modern high resolution devices and extensive available content a bettersolution is required. Modern devices support desktop style vector fonts allowing morecompatibility with content and better overall readability for the user, but at a cost of more CPUcycles. This cost is usually offset with caching schemes and other intelligent buffering, but thesein turn have their own limitations and complexities of implementation. If one considers thecurrent cycle of font rendering which requires conversion from the original glyph data into rasterbitmap information for display based on font face, style and size, followed by final per-pixelrendering, it is obvious there is potential for improvements. Although some support for GPUaccelerated vector fonts exists within OpenVG (since version 1.1) and also some modern desktopGPU hardware (with questionable quality [N1]), it is more advantageous to have access to theglyph rendering directly prior to the final rendering to utilize existing support for font effects,formatting, layout and other NUI engine specific capabilities. To provide some cutting edgevisual effects such as glowing or shadowed text the uSwish platform supports various textrendering engines for glyph support but handles pixel level drawing internally. Given the largeamount of content that requires text rendering, it is a great potential bottleneck for improvementfrom parallel computing. By processing the font data through the GPU the cost per glyph wouldbe much less especially if the complete cycle from vector data to final rendered image could beprocessed in a single pass without requiring data transfer back to the CPU. This solution wouldalso decrease the complexity of the font management since the requirement for font cachingschemes would be almost moot. Advanced font visual effects such as transitions and distortionswould also benefit from encapsulation into a single GPGPU processing cycle.Layout of NUI elements and content framesLayout is a common concern in development of a strong NUI solution and requires considerableconsideration of the traditional GUI counterparts since dynamic reorganization of content iscommon. Additional features such as device rotation from portrait to landscape view, usuallyvery cumbersome on traditional GUIs needs to be entertaining and immersive in a NUIenvironment. Transitions between UI modes or applications must be fluid and contextuallyuseful, requiring very fast layout recalculations. All this requires live updates at high frame ratesand applies well to a potential GPGPU implementation. Assumptions made in current CPU basedimplementations, such as handling large framed zones as single elements rather than eachelement within each zone can be removed. An example would be a free flowing transitionsbetween 2 view states in an email client, where individual elements (even down to the letters inthe email content) could be animated independently allowing for some very compelling visualeffects. Current CPU based implementations simply rotate the complete application display oranimate only portions of the display between states. As device resolution continues to increaseenhancing the amount of usable screen real estate there is an almost exponential increase in theamount of concurrent data presented to the user, demanding increased complexity of theunderlying layout handlers making it a prime candidate for GPGPU optimizations. 11

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Enhanced Visual featuresA common use of GPGPU functionality is to process image or visual effect data to achieveeffects either not supported by the GPU hardware, or not efficient in the normal OpenGL shaderparadigm. For example a particle system used to highlight areas of the UI would be fairly costlyin terms of the CPU processing, involving collision detection, motion calculations, and finallyutilizing the GPU for rendering. Using a GPGPU implementation the GPU could both processand render freeing the CPU for other operations, minimizing memory transfer and optimizingefficiency allowing for more particle elements or complex motion of the particles. An exampleof this type of particle system is detailed in “Heterogeneous Computing with OpenCL” [R1].Sorting of data or other data handling servicesData manipulation and sorting is a common theme in most content heavy applications. As anexample, many NUI based devices will provide summary data of SMS, email, tweets, etc. to theuser in an organized fashion for quick consumption of the data. The representation of this datacan be manipulated by sorting the events for date, importance, keywords and other user providedfilters. As the amount of available local storage space even on mobile devices is quite large thesorting process becomes a highly CPU intensive process leading to potential breaks in the userexperience and high CPU power consumption. By utilizing GPGPU processing we can performthese tasks much more efficiently. Given the latest generation of multi-core GPU devices wewould be able to utilize multiple cores to greatly reduce the time associated with this task.Compression / decompression of content storesThe high cost of memory transfer is a concern with large content stores and streaming data.Local content for NUI elements such as images, pre-rendered SVG content, etc., can be verycomplex or large depending on the visual impact intended for the user. We can see this exampleclearly in the Experience Station [*3] NUI implementation where dynamic controls are used forall user interactive elements. It is very common for GPU devices to support texture compressionto provide reduced memory footprint of texture data as stored on the local GPU RAM, howeverthe availability of GPGPU computing allows us to utilize compression of data stores of all localcontent that can be decompressed during the loading phase without reducing access time for thecontent. Expanding this capability through streaming data would allow for higher potentialbandwidth and reduction in cost of data usage. This long term application would require contentproviders to compress the content in a format compatible with the GPGPU based decompression.This method could be applied to content carrier formats or markup languages such as HTML.Today many web servers provide mobile versions of their content, yet next generation HTML5based servers could potentially detect the presence of a compression supported mobile deviceand relay the content in a compressed format, reducing bandwidth and data transfer costs. Thisprocess could be dynamic, allowing for optimization between bandwidth and data processingperformance of the target device.Physical properties of objects – reactive physics effects processing 12

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In order to provide a NUI experience it is crucial to provide very accurate “real world” physicalproperties to elements of the interface. Simple acceleration and velocity based calculations forscrolling lists are being replaced by more accurate mass, gravity, and inertia models oraccelerometer driven effects to provide a very compelling interaction for the user. As thecomplexity of NUIs increases to take advantage of the rendering capabilities of modern GPUs,the complexity of applied mathematics will increase, benefiting greatly from GPGPUacceleration.Complex animation or motion schemes for NUI visualsThe motion of elements in a NUI design can be very complex, requiring smooth easing motionsand high order Bezier curve based animation paths and transitions. These calculations can bevery costly as they must be performed for each drawn frame based on true elapsed time.Traditional optimization would involve pre-calculation or prediction algorithms to simplify thistask, but this is not always feasible in a true dynamic NUI experience. Since each elementaffected in the NUI requires individual calculation, the GPU SIMD architecture is a powerfulsolution which allows for full calculations in real time with minimal performance impact.Handling streaming NUI contentOne of the key areas of concern in an immersive NUI is the seamless flow of content to the user,both from local and remote stores. Dynamic control of content, whether simple text based data,images, video, or other forms should be handled smoothly in the background without interferingwith the user’s interactions. A good example is an image viewer application permitting the userto browse perhaps hundreds of photographs stored locally, many stored at very high resolution.In one example application developed using the uSwish NUI system, an almost infinite panel ofavailable images is provided in a grid format, allowing the user to freely navigate and quicklybrowse large amounts of photo content. Depending on number and layout of the images, veryhigh amounts of processing power are required to decode and display quickly enough to maintaina fluid natural flow to the user. Any operation which breaks the natural flow or motion results inan unresponsive feeling for the user. By making use of available GPU cycles to offloadtraditional CPU operations, the bandwidth demands can be decreased and the desired look andfeel of the interface can be maintained. Even with very high bandwidth demands and complexcontent stores, the user experience can be kept consistent.Real time security encryption / decryptionSecurity in mobile devices has always been a concern, and is becoming increasingly moreimportant as typical users migrate to using them as their main means of connectivity. Usingcomplex encryption algorithms for protecting sensitive information has been challenging at beston mobile platforms in the past. By using GPGPU capabilities, it would be possible to providemuch more secure encryption of local content without hindering the experience of the device forthe user.You I Labs uSwish engine specific features 13

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The uSwish platform utilizes a fully dynamic physics-enabled real time interface environmentwith full dynamic timeline animation control for a rich NUI experience. It is not uncommon forhigh levels of interconnected elements and effects to be manipulated during transitional eventswithin the scope of the interface. These complex interleaved sequences would benefit fromGPGPU acceleration as it would permit much higher numbers of connected objects to beprocessed and more complex computations to be performed at each step in the animationtimeline. Limitations and Cost of ImplementationThe potential benefits from using available GPU cycles for NUI enhancement are obvious: TheUI can be more visually appealing, increasingly interactive, yet smooth and responsive. Byutilizing GPGPU below the UI layer the platforms could benefit from more elaborate user inputsand gestures as well as support for new video or audio format decoders, expanding the useful lifeof the device as these standards evolve. Certainly these potential benefits and enhancementswould be nice to have, but at what cost?OpenCL or similar supportOne of the biggest challenges today is making use of the GPU within mobile devices. While theperformance for processing graphics is quite phenomenal, support for OpenCL or similar vendorspecific APIs is not always available. Making use of existing OpenGL 2.0ES graphics APIs toachieve complex GPGPU functionality is very cumbersome and costly to implement. Anotherconcern is the performance of functionality crucial to GPGPU applications that typically ishindered on mobile platforms: Frame Buffer Object (FBO) support and pixel read-backperformance. Both of these factors greatly limit the feasibility of using the GPU for outsidetasks. We must realize that the throughput of the GPGPU kernel is only one part of the equation:we must consider the costs of transferring data from CPU to GPU space and back, which can bevery costly. Even if the complexity of the calculations and amount of data to process makethroughput on the GPU seem factors above the comparable CPU implementation, it may benegated by memory transfer speed and other hardware factors.Code complexity:When working with large data sets such as encoded video streams, it is typical to optimize theimplementation to always utilize the GPU pipeline due to the nature of the content. The samecannot be said for NUI elements where some operations may cross the boundary betweenrequiring GPU performance and actually being hindered by the overhead of the GPGPU process. 14

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In these cases intelligent decisions must be made as to CPU versus GPU for best case dataprocessing. This requires that a proper NUI framework support both implementation methods,adding development costs, code complexity, and associated costs.Sharing Device Resources:Another major concern is the use of the available resources on the platform by external devicesand applications. A NUI design which makes heavy use of available GPU cycles may encounterusability issues when resources are not available. A device using a complex gesture recognitionsystem may need heavy GPU cycles or have stringent timing constraints that make the use ofGPU optimized UI elements simply not accessible while it is running. Applications that expectfull GPU cycles to be available may not perform correctly if cycles are taken by the NUI forbackground tasks or data / content processing. 15

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Conclusions As multiple processing core devices become more prevalent in the market, the need forparallel computing and intelligent use of the CPU resources has become paramount. Platformswith multiple core CPUs are now commonplace, even in the mobile space. GPUs themselves areno longer simply fast polygon renderers but powerful computational blocks capable of providingtremendous value in offsetting CPU load for certain functions. Natural User Interfaces requirelarge amounts of processing power to provide users with a fluid experience and will easilybenefit from the potential performance and efficiency offered by utilizing available GPU cycles.Existing limitations which complicate implementation today, such as support for standards likeOpenCL, are quickly being rectified. As mobile devices migrate from being peripheral todesktop computers to effectively replacing them, processing performance must scale to meet userdemand while still maintaining efficiency to ensure long battery life. By utilizing GPGPU inNUI applications we can maintain the best user experience by maximizing the hardware’s fullcapabilities. 16